A) Field of the Invention
The present invention relates to semiconductor devices using a silicon oxynitride film as a gate insulating film, to manufacture and evaluation methods for such semiconductor devices, and to a process condition evaluation method of evaluating the conditions of a silicon oxynitride film forming process.
B) Description of the Related Art
A gate insulating film of a complementary MOS field effect transistor (CMOSFET) becomes thinner as the semiconductor integrated circuit device is made smaller and highly integrated. As a gate insulating film is made too thin, boron atoms implanted into the gate electrode of a p-channel MOSFET may pass through the gate insulating film and diffuse into the channel region. This boron diffusion (boron drainage) can be prevented by using, as the material of a gate insulating film, silicon oxynitride made by introducing nitrogen into silicon oxide. In introducing nitrogen atoms into silicon oxide, annealing is generally performed in a gas atmosphere of ammonium (NH3), NO or N2O by using a heat treatment system of a resistance heating type or a lamp heating type.
A method using nitrogen plasma has been proposed recently to introduce more nitrogen atoms. The effects of preventing boron drainage can be enhanced by raising the nitrogen density in a gate insulating film. As the nitrogen density is raised, the dielectric constant of the gate insulating film becomes higher than that of a silicon oxide film. The same electrostatic capacitance can therefore be retained by thickening the gate insulating film. This results in a small gate leak current.
As nitrogen is introduced by using a heat treatment system of a resistance heating type or a lamp heating type, the nitrogen density distribution has a peak near at the interface between a silicon oxynitride film and a silicon substrate. As nitrogen is introduced by using nitrogen plasma, the nitrogen density distribution has a peak in the central area in the depth direction of a silicon oxynitride film or in an area nearer to the surface of the film than the central area. By using nitrogen plasma, the electrical characteristics (carrier mobility and the like) and reliability of a metal insulator semiconductor FET (MISFET) can be improved because the nitrogen density in the gate insulating film can be raised without raising the nitrogen density near at the interface between the gate insulating film and the channel.
It is known that the mutual conductance (Gm) of MISFET, which is one index indicative of a MISFET performance, is degraded depending upon a method of introducing nitrogen. Refer to the related technical paper: “Downscaling Limit of Equivalent Oxide Thickness in Formation of Ultrathin Gate Dielectric by Thermal-Enhanced Remote Plasma Nitridation” by Chien-Hao Chen, IEEE TRANSACTIONS ON ELECTRON DEVICES, MAY 2002, VOL. 49, NO. 5. p. 840-845.
Japanese Patent Laid-open Publication No. HEI-11-204787 discloses the preferable structure of a gate insulating film which can realize high speed and high reliability MISFET. This gate insulating film is made of silicon oxynitride, and nitrogen atoms distribute only near at the interface between the silicon substrate and gate insulating film. Near at the interface, each of all nitrogen atoms is coupled to two silicon atoms and one oxygen atom or to three silicon atoms.
A plasma nitridation process is difficult to form the gate insulating film disclosed in this Publication. This is because the plasma nitridation process introduces nitrogen into the gate insulating film in an area away from the interface between the silicon substrate and gate insulating film or in an area near at the surface of the gate insulating film. It is also difficult to distribute nitrogen atoms only near at the interface between a gate insulating film and a silicon substrate if the gate insulating film becomes thinner.
There are a variety of gate insulating film forming methods. For the earlier entry of better performance semiconductor devices into markets, it is required to clarify the relation between the nitridation state of a gate insulating film and a MISFET performance index such as a mutual conductance (Gm). If the relation therebetween is clarified, analyzing a silicon oxynitride film used as a gate insulating film of MISFET makes it possible to estimate the performance of MISFET.